Turbo-molecular pumps which are similar in structure to turbines but which resemble the Gaede molecular pump in their operating principle, that is, that their compression energy finds its source in the shock of the molecules against the walls in motion, have been known for a long time.
It is also known that this type of pump is, at present, the object of great development in the field of vacuum techniques, in which these pumps are used as secondary pumps because of their aptitude to constitute an effective barrier against oil vapors which may be retrodiffused from the primary vacuum.
It is also known that the compression ratio per stage of a molecular pump may be evaluated by means of a mathematical formula expressing the fact that the logarithm of the compression ratio per stage is proportional to the square root of the molecular mass of the pumped gas. The result of this is that these pumps have remarkable effectiveness for the removal of heavy gases, such as, more particularly, oil vapors, but that, on the other hand, their performance is very slight in contact with a light gas, such as hydrogen which, it is known, is always found in great proportions in the residual atmosphere of enclosures in which a vacuum is formed.
In this way, for example, a turbo-molecular pump having 16 stages with a compression ratio of 10.sup.8 for nitrogen would cause a perfectly, satisfactory removal of the heavy vapors such as oil vapors, but it can be easily calculated that the compression ratio of hydrogen would only be in the vicinity of 10.sup.2, this being clearly insufficient in a great number of cases.
It is very easy to observe that, to reach the same compression ratio as for nitrogen, it would be necessary to have available 3.7 times as many stages. That is, a pump would require 59 or 60 stages, and this is prohibitive.
Even if certain users can accept the use of such equipment, its price would be all the higher for those users who only need to remove nitrogen in which a turbo-molecular pump having 16 stages is preferred. In this way, the constructor would be induced to produce very small series of pumps having a varied number of stages corresponding to the requirements of each to be used; and in conclusion, turbo-molecular pumps are not very easy to adapt to the kind of gas to be pumped and their manufacturing cost remains high.
But it is known on the other hand that these turbo-molecular pumps have the great advantage of a very substantially constant output whatever the molecular mass of the gases pumped may be.
It, therefore, appears to be an advantage to combine these properties of turbo-molecular pumps capable of pumping gases having different molecular masses with the same output, but at a different compression ratio, with the properties of rotating drum type molecular pumps in which the manufacturer can increase at will the compression ratio simply by modifying the depth of the grooves.
Indeed, it is known that rotating drum type molecular pumps comprise a cylindrical drum rotating at high speed with slight clearance inside a stator whose inside face is also cylindrical. On the inside face of the stator, on the outside face of the drum, or on both the two adjacent faces, several parallel grooves are formed having a helical shape whose depth, decreasing from the inlet to the output, determines the compression ratio for a given gas, and whose cross-section determines the output.
It is known that this output is, as a general rule, less than the required output. However, on connecting up the suction part of such a rotating drum type molecular pump to the outlet of a turbo-molecular pump, whose output is satisfactory, the output in weight of the rotating drum type molecular pump is improved since the latter will have an effect on a gas whose volume has been reduced in a proportion equal to the compression ratio of the turbo-molecular pump. The advantage thus obtained is very clear, but it may nevertheless be considered insufficient or too expensive by users if the two pumps are not perfectly adapted to each other and do not meet certain requirements. Thus, on examining again the preceding example, and if the molecular pump having 16 stages providing a compression ratio of 10.sup.8 for nitrogen and about 100 for hydrogen is combined with a drum type molecular pump whose depth has been especially calculated for hydrogen, it will be possible to produce a total compression ratio, for hydrogen, of 10.sup.3, but the price of such an equipment will remain prohibitive since the price of the complete turbo-molecular pump is added to that of the rotating drum type pump.
It may be conceived that if a structure affording an advantage, combining these two types of pumps is required to be produced, it is necessary to adapt the two types of pumps to each other by harmoniously distributing the compression ratio to be established between the two components, so as to improve the discharge in weight of the part of the structure fulfilling the function of a rotating drum type molecular pump.
In order to reduce the size and cost of a structure combining a turbo-molecular pumping element with a rotating drum type pumping element, it is considered in the present invention an advantage to contrive a standard element in which the number of stages of the turbo-molecular element has as low a value as possible, making it possible, nevertheless, to obtain a substantial improvement in the output in weight of the rotating drum type molecular pumping element.